Geometry in Pascal

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Checking if a point is inside a polygon (integer version)

//  The function will return True if the point x,y is inside the polygon, or
//  False if it is not.
//
//  Original C code: http://www.visibone.com/inpoly/inpoly.c.txt
//
//  Translation from C by Felipe Monteiro de Carvalho
//
//  License: Public Domain
function IsPointInPolygon(AX, AY: Integer; APolygon: array of TPoint): Boolean;
var
  xnew, ynew: Cardinal;
  xold,yold: Cardinal;
  x1,y1: Cardinal;
  x2,y2: Cardinal;
  i, npoints: Integer;
  inside: Integer = 0;
begin
  Result := False;
  npoints := Length(APolygon);
  if (npoints < 3) then Exit;
  xold := APolygon[npoints-1].X;
  yold := APolygon[npoints-1].Y;
  for i := 0 to npoints - 1 do
  begin
    xnew := APolygon[i].X;
    ynew := APolygon[i].Y;
    if (xnew > xold) then
    begin
      x1:=xold;
      x2:=xnew;
      y1:=yold;
      y2:=ynew;
    end
    else
    begin
      x1:=xnew;
      x2:=xold;
      y1:=ynew;
      y2:=yold;
    end;
    if (((xnew < AX) = (AX <= xold))         // edge "open" at left end
      and ((AY-y1)*(x2-x1) < (y2-y1)*(AX-x1))) then
    begin
      inside := not inside;
    end;
    xold:=xnew;
    yold:=ynew;
  end;
  Result := inside <> 0;
end;

See also